Full Moon

From Conservapedia

A Full Moon is the phase of the Moon when it is fully illuminated as a circle. Full Moons occur approximately every 28 days. Lunar eclipses may only occur during a Full Moon, while solar eclipses may occur only during a new moon.

Two full moons in a month - a fairly rare occurrence (every 32 months) - is called a "blue moon," leading to the phrase "once in a blue moon" to describe things that happen rarely.